the last active page

This fixes a bug introduced in commit 8365f2f7 and improves the previous,
suboptimal behaviour. Now, polygon fill rules should be saved and loaded
correctly; and the changes should be reverse compatible (so that files
created with an older version will be loaded correctly as well).

(finished) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups

(WIP) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups'

fixed a bug where clicking on/moving a textbox by its rubberband whith stylus tool on text caused a crash (more frequent after zoom out)

fixed a bug where thumbnail view was not centering on the active thumbnail view, when clicking on the Next Icon (main bar)

Revert "workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes"

This reverts commit 0fd15f62.

workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes

This reverts commit b6f41165.